Outside the larger cities, your choice of food and restaurant will be quite restricted. In Dhaka there are some excellent restaurants serving Thai, Chinese, Italian, and other cuisines, but in the countryside you will only find basic restaurants and local food. Local food has been influenced by history, geography, and religion--the Mughals, the Indians, and the Muslims.
A typical rural meal will consist of rice and a curry made from beef, mutton, chicken, fish or egg, cooked in a spicy sauce and served with cooked yellow lentils (dahl). After a while you will start to get sick of it. This is when you will explore other things which may be available. Look for Bhuna, which is meat fried over high heat served with a spicy sauce. Bhaji is fried beans, squash, or some other vegetable. Khichuri is rice and lentils cooked together. Tikka is sometimes available and is usually chicken with sauce served with Indian-style naan bread. Fish is sometimes available with Bhetki, a sea bass, one of the best species. Bread and biscuits are available everywhere, and at times this will be all that appeals. Eat them with the local milky sweet tea and you will survive for another day.
Drinks can be a problem. Bottled water is usually available, but there is no guarantee that the bottle has not been refilled using unsafe water. International soft drinks are readily available, and some local brands at half the price compete with these. Green coconut juice is safe if you see the coconut being opened, and this can be helpful in treating diarrhoea, which you will catch at some time during your Bangladeshi travels. Alcoholic drinks are not usually available, and I advise against trying the local brews you may be offered in some areas.
Traditionally, meals are served on the floor with each person sitting on a small piece of square carpet, but this is now rare in restaurants. Instead you will sit at rickety tables and probably be expected to eat with your hands. Don’t be concerned about manners, as you will notice vastly different eating styles and plenty of slurping, burping, and playing with the food. Always wash your hands before you eat, never put food into your mouth with the left hand, and have fun.
You can finish a meal with some fruit (oranges, bananas, and mangoes are available for many months of the year) or some sweetened yogurt (misti doi). Lassi, the great yogurt drink very common in India, Tibet, and Nepal, is sometimes available, but I have never really enjoyed the Bangladeshi version. Perhaps you will have more luck.